The invention relates to a two-stroke engine such as a drive engine in a portable handheld work apparatus including a motor chain saw, brushcutter, cutoff machine or the like.
In known two-stroke engines, the air/fuel mixture needed for operation as well as clean air are supplied to the crankcase. The air flows in via channels close to the outlet. The entry openings of the transfer channels in the crankcase housing are arranged at different spatial locations in order to ensure that air or a low-fuel mixture enters the combustion chamber from the crankcase via the outlet-near transfer channel and that only a rich air/fuel mixture enters via the outlet-remote channel. This requires a complex channel arrangement and ensures that only air is supplied via the outlet-near channel at the start of the scavenging cycle. After a first introduction of air, a low-fuel mixture flows from the crankcase which escapes in considerable amounts via the outlet because of the outlet-near position of the transfer channels. This leads to an excellent charge of the combustion chamber but causes high hydrocarbon emissions in the exhaust gas because of the energy-rich scavenging losses which is unacceptable in view of a need to provide a high measure of environmental compatibility.
It is an object of the invention to provide a two-stroke engine which is improved so that, on the one hand, a complete charge of the combustion chamber with a mixture is ensured while, on the other hand, the mixture component, which escapes via the outlet, is held as small as possible.
The two-stroke engine of the invention includes a two-stroke engine in a portable handheld work apparatus. The two-stroke engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; a carburetor for supplying an air/fuel mixture and the carburetor having an intake channel; an inlet channel connected to the intake channel and leading to the crankcase for conducting the air/fuel mixture into the crankcase; the cylinder having a discharge outlet formed therein for conducting exhaust gases away from the combustion chamber; an outlet-near transfer channel connecting the crankcase to the combustion chamber; the outlet-near transfer channel having a first end defining a transfer window opening into the combustion chamber and a second end defining inflow opening open to the crankcase; an air channel connected to transfer channel between the first and second ends thereof for supplying an essentially fuel-free gas flow to the transfer channel; an outlet-remote transfer channel connecting the crankcase to the combustion chamber; the outlet-remote transfer channel having a first end defining a transfer window opening into the combustion chamber and a second end defining and inflow opening open to the crankcase; and, a sum of the constructive volumes of the outlet-near transfer channel between the transfer window and the inflow opening being approximately 20% to 60% of the volumetric total air input of the engine at rated engine speed.
The dimensions of the outlet-near transfer channels (that is, the constructive volumes between the transfer windows of the channels to the combustion chamber and their inflow openings from the crankcase) are so predetermined that the sum of these constructive volumes corresponds to approximately 20% to 60% of the total volumetric air input of the engine at rated engine speed rpm. In this way, a significant part of the total air of the engine is supplied as pure air via the outlet-near transfer channels. The mixture, which flows in from the crankcase, is designed to be correspondingly rich so that, after closing the outlet, the remaining air and the rich mixture define an air/fuel ratio which ensures a substantially complete combustion for a ready power development. The air quantity, which flows in via the outlet-near transfer windows corresponding to the provided constructive volume, provides an air curtain, which shields the outlet over the long duration of the scavenging cycle. This air curtain prevents an escape of the rich air/fuel mixture. Preferably, toward the end of the scavenging phase, the air/fuel mixture, which is inducted into the crankcase, can follow on also via the outlet-near transfer channels. For this reason, an advantageous change of the charge stratification results for the next combustion.
The outlet-near transfer channel is configured so as to be closed to the piston in order to easily make available the constructive volume of the outlet-near transfer channel. In this way, a wall is formed between a transfer channel and the cylinder bore and the thickness of the cylinder wall is between 2 mm and 6.5 mm. This thickness ensures an adequate shielding of the transfer channel from the hot interior of the cylinder whereby an excessive heating of the advanced air in the outlet-near transfer channel is prevented.
The outlet-remote transfer channels have the exclusive function of transfer. For this reason, these outlet-remote transfer channels are configured in a simple manner to be open toward the cylinder bore and this reduces manufacturing complexity. It is practical to also configure the outlet-remote transfer channels as closed toward the cylinder bore.
The two-stroke engine of the invention is advantageously so operated that approximately 30% to 70% (preferably approximately 35% to 45%) of the volumetric total air input of the engine is supplied via the outlet-near transfer channel at rated rpm of the engine. The volume design for the outlet-near transfer channels is provided in such a manner that the stratified charge, which is formed in the combustion chamber, is maintained over approximately 65% to 95% (preferably approximately 75%) of the duration of the scavenging operation.